A pregnant Sumatran rhinoceros is expected to give birth soon at a sanctuary in Indonesia, in a rare event that has only happened three times in the last century, experts said Friday. \"It could be any day now,\" said Bill Konstant, program officer at the US-based International Rhino Foundation, which has been working with veterinarians to support the pregnancy of Ratu, a 12-year-old Sumatran rhino. Sumatran rhinos are critically endangered and have suffered a 50 percent drop in population over the last 20 years, largely due to poaching and loss of tropical habitat. There are now believed to be fewer than 200 Sumatran rhinos alive. Most reside in isolated pockets in Southeast Asia. The last three in-captivity births for Sumatran rhinos took place in the United States at the Cincinnati Zoo in Ohio. One of those was a male named Andalas, born September 13, 2001. He was raised in captivity and was recently brought to Indonesia to mate with Ratu, a female who grew up in the wild but wandered out of the forest and now lives at the Sumatran Rhino Sanctuary in Way Kambas National Park. This is believed to be Ratu\'s first full-term pregnancy, Konstant told AFP. She has already miscarried twice after prior attempts to breed in captivity. But experts are hopeful for good news soon. Ratu has been pregnant for 16 months and her calf is expected to arrive any time in the next two to three weeks. \"Ratu\'s pregnancy gives hope to the conservation of the endangered Sumatran rhino population and our whole team is excited to be a part of this moment in conservation history,\" said Dedi Candra, head veterinarian at the Sumatran Rhino Sanctuary.